# Constant lunar sub-surface temperature

On Earth we have the idea of constant ground temperature. Builders in North America think of it as 55 degrees at 5 feet down (not quite true, but close enough). At the surface it varies with the day. A little deeper and it averages the week. The deeper you go, the more it lags behind until you get to 30 feet where it is constant, roughly equal to the annual average of the area.

In considering an underground lunar settlement, the constant ground temperature in the first 60 meters or so, where you might tunnel is a really relevant. Since the lunar day is so long and the temperature varies so much, one would think the depth at which constant temperature occurs would be deeper than earth. But maybe soil in a vacuum transmits heat less? I think it would probably vary a lot by latitude as well. The thought of having an underground settlement where the ground temperature is naturally comfortable to people is fascinating.

Do we know what the constant lunar underground temperature is? And at what depth it stabilizes? Is there any place on the moon where it might be in the 40˚ to 80˚F range?

• Without atmosphere, equilibrium temperature at 1AU is about -17C. I would expect that to be the temp "near" the surface around the equator. Colder as the latitude increases. Commented Jan 26, 2017 at 4:36
• I'm not sure if this is a duplicate of Does the temperature near the surface of the Moon rise rather quickly with depth?. It comes at the matter from a different tack, but the needed information would seem to be the same. Commented Jan 26, 2017 at 15:09
• There is an odd discrepancy there with Apollo giving hotter measurements. The Indian Chandrayaan-1 probe spotted an apparent lava tube and somehow estimated a temperature of -5˚f So this fits with the -17C statement above. But it is much more complex than that. The moon is not active but the center it is still hot. There is surely an increasing gradient that starts somewhere. (Rock pressure is probably prohibitably high at that point anyway.) Commented Jan 26, 2017 at 22:27

tl;dr: The number is often given as 252 +/- 3 K (or about -21°C or -6°F), so @BowlOfRed pretty much nailed it using first principles!

Without atmosphere, equilibrium temperature at 1AU is about -17C. I would expect that to be the temp "near" the surface around the equator. Colder as the latitude increases.

Reading the references, it seems that the diurnal oscillation (what Earthies might call "monthly" or Lunies might call "daily") in temperature decays with a 1/e depth of about 30 centimeters, so by the time one reaches 1 meter, the temperature "stabilizes" temporally, but of course will increase with depth since the Moon's interior is hot. You can see this in Figure 2 of the last paper ("Subsurface temperature histories, covering 3.5 years...") where the oscillations in temperature at different depths along the probe are plotted.

Further, some of this data can already be found in @KimHolder's question Does the temperature near the surface of the Moon rise rather quickly with depth? who quotes as NASA News article Down the Lunar Rabbit-hole which then quotes Mark Robinson of Arizona State University, principal investigator for the LRO camera.

"The tunnels offer a perfect radiation shield and a very benign thermal environment," says Robinson. "Once you get down to 2 meters under the surface of the Moon, the temperature remains fairly constant, probably around -30 to -40 degrees C."

Why the difference between the "ground truth" actual temperature measurements discussed below and the quote of a quote?

(image source also here) My guess is that the -30 to -40 number is a mid-latitude number, while the following concentrates on nearer to equator values. There's no physical way the temperature can drop by 10 or 20C in 1 meter of increasing depth, so the difference is more context and location.

I'm not an expert on planetary science, but the oft-cited number for an average sub-surface temperature about 1 meter below the surface seems to come from the following papers:

1. The Apollo 15 lunar heat-flow measurement, Langseth, M.G., Clark, S.P., Chute, J.L. et al. The Moon (1972) 4: 390. https://doi.org/10.1007/BF00562006

Abstract:

The heat-flow experiment is one of the Apollo Lunar Surface Experiment Package (ALSEP) instruments that was emplaced on the lunar surface on Apollo 15. This experiment is designed to make temperature and thermal property measurements in the lunar subsurface so as to determine the rate of heat loss from the lunar interior through the surface. About 45 days (1 1/2 lunations) of data has been analyzed in a preliminary way. This analysis indicates that the vertical heat flow through the regolith at one probe site is 3.3 × 10−6 W/cm2 (±15%). This value is approximately one-half the Earth's average heat flow. Further analysis of data over several lunations is required to demonstrate that this value is representative of the heat flow at the Hadley Rille site. The mean subsurface temperature at a depth of 1 m is approximately 252.4K at one probe site and 250.7K at the other. These temperatures are approximately 35K above the mean surface temperature and indicate that conductivity in the surficial layer of the Moon is highly temperature dependent. Between 1 and 1.5m, the rate of temperature increase as a function of depth is 1.75K/m (±2%) at the probe 1 site. In situ measurements indicate that the thermal conductivity of the regolith increases with depth. Thermal-conductivity values between 1.4 × 10−4 and 2.5 × 10−4 W/cm K were determined; these values are a factor of 7 to 10 greater than the values of the surface conductivity. If the observed heat flow at Hadley Base is representative of the moonwide rate of heat loss (an assumption which is not fully justified at this time), it would imply that overall radioactive heat production in the Moon is greater than in classes of meteorites that have formed the basis of Earth and Moon bulk composition models in the past.

1. In-situ measurements of lunar heat flow (also here) in Soviet-American Conference on Geochemistry of the Moon and Planets, Langseth, M. G., and S. J. Keihm, NASA Spec. Publ., NASA SP-30, 283 – 293, 1977.

The number is often given as 252 +/- 3 K (or about -21°C or -6°F) as for example in Latitude variation of the subsurface lunar temperature: Lunar Prospector thermal neutrons R. C. Little W. C. Feldman S. Maurice I. Genetay D. J. Lawrence S. L. Lawson O. Gasnault B. L. Barraclough R. C. Elphic T. H. Prettyman A. B. Binder First published: 29 May 2003 https://doi.org/10.1029/2001JE001497

Some of the experimental and theoretical details behind the data and its analysis can be read in Revised lunar heat-flow values, Langseth, M. G., Keihm, S. J., & Peters, K., Lunar Science Conference, 7th, Houston, Tex., March 15-19, 1976, Proceedings. Volume 3. (A77-34651 15-91) New York, Pergamon Press, Inc., 1976, p. 3143-3171.

• Could this paper be a motive to update your answer ? agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JE005579 Commented Dec 22, 2021 at 11:01
• @Cornelis It looks like a very thorough analysis which means it will take some time to read. Thanks! fyi there's a typo in your url; double-dots before "wiley"
– uhoh
Commented Dec 22, 2021 at 11:36
• Sorry, now that I've read the paper, with the findings there I think there's no need for "updating" . But it might be interesting for you anyway. Commented Dec 22, 2021 at 16:08